Audio systems for providing isolated listening zones

ABSTRACT

An audio system includes a plurality of near-field speakers arranged in a listening area. A plurality of cross-talk cancellation filters are coupled to the speakers. The speakers and the filters are arranged to provide first and second listening zones in the listening area such that audio from the first listening zone is cancelled in the second listening zone and vice versa. The system also includes at least one audio source providing audio content. Volume-based equalization circuitry receives an audio signal representing audio content for the first listening zone from the audio source and controls a volume adjustment applied to the audio signal to control a volume of audio in the first listening zone. The circuitry limits attenuation or amplification of a first frequency portion of the audio signal when a volume setting differential corresponding to a difference between volume settings for the first and second zones exceeds a predetermined value.

BACKGROUND

This disclosure relates to audio systems for providing isolatedlistening zones

SUMMARY

All examples and features mentioned below can be combined in anytechnically possible way.

In one aspect, an audio system includes a plurality of near-fieldspeakers arranged in a listening area. A plurality of cross-talkcancellation filters are coupled to the near-field speakers. Thenear-field speakers and the cross-talk cancellation filters are arrangedto provide first and second listening zones in the listening area suchthat audio from the first listening zone is cancelled in the secondlistening zone and vice versa. The audio system also includes at leastone audio source for providing audio content. First volume-basedequalization circuitry is configured to receive a first audio signalrepresenting audio content for the first listening zone from at leastone audio source. The first volume-based equalization circuitry controlsa volume adjustment applied to the first audio signal thereby to controla volume of audio in the first listening zone based on a first volumesetting. Second volume-based equalization circuitry is configured toreceive a second audio signal representing audio content for the secondlistening zone. The second volume-based equalization circuitry controlsa volume adjustment applied to the second audio signal thereby tocontrol a volume of audio in the second listening zone based on a secondvolume setting. The first volume-based equalization circuitry isconfigured to limit attenuation or amplification of a first frequencyportion of the first audio signal when a volume setting differentialcorresponding to a difference between the first and second volumesettings exceeds a predetermined value.

Implementations may include one of the following features, or anycombination thereof.

In some implementations, the second volume-based equalization circuitryis configured to limit attenuation or amplification of a secondfrequency portion of the second audio signal when the volume settingdifferential exceeds a predetermined value.

In certain implementations, the first frequency portion and the secondfrequency portion are the same.

In some cases, the second frequency portion is a low frequency portion,and the second volume-based equalization circuitry is configured tolimit attenuation of the low frequency portion of the second audiosignal when the volume setting differential exceeds a predeterminedvalue.

In certain cases, the audio system includes control circuitry that isconfigured to provide the first and second volume-based equalizationcircuitry with a volume control signal. The first volume-basedequalization circuitry is configured to determine the volume settingdifferential based on information provided in the volume control signal.

In some examples, the first volume-based equalization circuitry onlylimits the attenuation or amplification of the first frequency portionof the first audio signal based on the volume setting differential whenthe first and second audio signals represent the same audio content.

In certain examples, the first frequency portion is a low frequencyportion, and the first volume-based equalization circuitry is configuredto limit attenuation of the low frequency portion of the first audiosignal when a volume setting differential corresponding to a differencebetween the first and second volume settings exceeds a predeterminedvalue.

In another aspect, an audio system includes a plurality of near-fieldspeakers arranged in a listening area, and a plurality of cross-talkcancellation filters coupled to the near-field speakers. The near-fieldspeakers and the cross-talk cancellation filters being arranged toprovide first and second listening zones in the listening area in whichaudio from the first listening zone is cancelled in the second listeningzone and vice versa. The audio system also includes a plurality ofsensors for detecting occupancy within the listening area. A filtercoefficient determination module is configured to select filtercoefficients for the cross-talk cancellation filters based on inputindicative of the occupancy obtained via the plurality of sensors.

Implementations may include one of the above and/or below features, orany combination thereof.

In some implementations, the audio system also includes a look-up tablethat stores the filter coefficients, and the filter coefficientdetermination module selects the filter coefficients from the look-uptable.

Another aspect features an audio system that includes a plurality ofnear-field speakers, and a plurality of cross-talk cancellation filterblocks. The plurality of near-field speakers include first, second, andthird sets of near-field speakers. The plurality of cross-talkcancellation filter blocks include first, second, third, and fourthcross-talk cancellation filter blocks. The first cross-talk cancellationfilter block includes a first plurality of cross-talk cancellationfilters for filtering a first audio signal to provide first filteredaudio signals which are transduced by the first set of near-fieldspeakers to present audio content of the first audio signal at one ormore seating locations in the first listening zone and to provideinter-aural cross-talk cancellation of left channel and right channelaudio content of the first audio signal at the one or more seatinglocations in the first listening zone. The second cross-talkcancellation filter block includes a second plurality of cross-talkcancellation filters for filtering the first audio signal to providesecond filtered audio signals which are transduced by the second set ofnear-field speakers to cancel audio content of the first audio signal atone or more seating locations in the second listening zone. The thirdcross-talk cancellation filter block includes a third plurality ofcross-talk cancellation filters for filtering a second audio signal toprovide third filtered audio signals which are transduced by the firstset of near-field speakers to cancel audio content of the second audiosignal at the one or more seating locations in the first listening zone.The fourth cross-talk cancellation filter block includes a fourthplurality of cross-talk cancellation filters for filtering the secondaudio signal to provide fourth filtered audio signals which aretransduced by the third set of near-field speakers to present audiocontent of the second audio signal at the one or more seating locationsin the second listening zone and to provide inter-aural cross-talkcancellation of left channel and right channel audio content of thesecond audio signal at the one or more seating locations in the secondlistening zone.

Implementations may include one of the above and/or below features, orany combination thereof.

In some implementations, the first and second sets of near-fieldspeakers are mounted in one or more headrests in a first row of seats inthe listening area, and the third set of near-field speakers are mountedin one or more headrests in a second row of seats positioned behind thefirst row of seats in the listening area. The one or more seatinglocations in the first listening zone are in the first row of seats,and, the one or more seating locations in the second listening zone arein the second row of seats.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of an implementation of a vehicle audiosystem for providing isolated listening zones in a vehicle cabin.

FIGS. 2A-2D are schematic views of cross-talk cancellation filter blocksand associated headrest mounted speakers from the vehicle audio systemof FIG. 1.

FIG. 3 is a schematic view of a second implementation of a vehicle audiosystem for providing isolated listening zones in a vehicle cabin.

FIG. 4 is a schematic view of volume-based equalization circuitry fromthe vehicle audio system of FIG. 3.

FIG. 5 is graphical representation of attenuation curves that may beapplied by the volume-based equalization circuitry of FIG. 4.

FIG. 6 is a schematic view of a third implementation of a vehicle audiosystem for providing isolated listening zones in a vehicle cabin.

FIG. 7 is a schematic view of a fourth implementation of a vehicle audiosystem for providing isolated listening zones in a vehicle cabin.

FIGS. 8A and 8B are schematic views of cross-talk cancellation filterblocks and associated headrest mounted speakers from the vehicle audiosystem of FIG. 7.

FIG. 9 is a schematic view of a fifth implementation of a vehicle audiosystem for providing isolated listening zones in a vehicle cabin.

FIG. 10 is a schematic view of a second implementation of volume-basedequalization circuitry.

Repeat use of reference characters in the present specification anddrawings is intended to represent same or analogous features orelements.

DETAILED DESCRIPTION

Though the elements of several views of the drawings herein may be shownand described as discrete elements in a block diagram and may bereferred to as “circuitry,” unless otherwise indicated, the elements maybe implemented as one of, or a combination of, analog circuitry, digitalcircuitry, or one or more microprocessors executing softwareinstructions. The software instructions may include digital signalprocessing (DSP) instructions. Unless otherwise indicated, signal linesmay be implemented as discrete analog or digital signal lines, as asingle discrete digital signal line with appropriate signal processingto process separate streams of audio signals, or as elements of awireless communication system. Some of the processing operations may beexpressed in terms of the calculation and application of coefficients.The equivalent of calculating and applying coefficients can be performedby other analog or digital signal processing techniques and are includedwithin the scope of this patent application. Unless otherwise indicated,audio signals may be encoded in either digital or analog form;conventional digital-to-analog or analog-to-digital converters may notbe shown in the figures. For simplicity of wording, “radiating acousticenergy corresponding to the audio signals” in a given channel or from agiven array will be referred to as “radiating” the channel from thearray.

Cross-talk cancellation can be utilized in combination with near-fieldspeakers to provide discrete listening zones at different seatingpositions within a listening area, such as a vehicle cabin. As usedherein “near-field speakers” is intended to mean speakers located near alisteners head. FIG. 1 illustrates an exemplary implementation of avehicle audio system 100 that incorporates a number of cross-talkcancellation filters in combination with a plurality of headrest mountedspeakers to provide two discrete (front and rear) listening zones 101 a,101 b.

The system 100 includes one or more audio sources 102 which are coupledto audio signal processing circuitry 104. The audio signal processingcircuitry 104 is coupled to front and rear volume adjustment circuitry106 a, 106 b, respectively. The front and rear volume adjustmentcircuitry 106 a, 106 b is coupled to the headrest mounted speakers viacross-talk cancellation filter blocks 110 a-d.

In response to control information received from a user through manualinput, a control circuit 114 sends a signal 116 to the audio signalprocessing circuitry 104 selecting a given audio source for the frontand rear listening zones 101 a, 101 b. That is, the signal identifieswhich audio source is selected for each of the listening zones. Eachlistening zone can select a different audio source, or a common audiosource may be selected for both of the front and rear listening zones101 a, 101 b.

The audio signal processing circuitry 104 delivers a first audio signal118 representing audio content for the front zone 101 a to the frontvolume adjustment circuitry 106 a, and delivers a second audio signal120 representing audio content for the rear zone 101 b to the rearvolume adjustment circuitry 106 b.

In response to volume control information received from a user throughmanual input, the control circuit sends first and second volume controlsignals 122, 124 to the front and rear volume adjustment circuitry 106a, 106 b, respectively. The front and rear volume adjustment circuitry106 a, 106 b adjust the respective amplitudes of the first and secondaudio signals 118, 120 in response to the volume control signals 122,124 and provide the amplitude adjusted audio signals 126, 128 to thecross-talk cancellation filter blocks 110 a-d. In that regard, the frontvolume adjustment circuitry 106 a controls volume of audio contentpresented in the front listening zone 101 a, and the rear volumeadjustment circuitry 106 b operates to control the volume of audiocontent presented in the rear listening zone 101 b. Consequently, evenwhen the same audio content is selected for presentation in both zones,the volume level may still differ between the zones.

In the illustrated example, the front volume adjustment circuitry 106 aprovides a first amplitude adjusted audio signal 126 to first and secondcross-talk cancellation filter blocks 110 a, 110 b, and the rear volumeadjustment circuitry 106 b provides a second amplitude adjusted audiosignal 128 to third and fourth cross-talk cancellation filter blocks 110c, 110 d.

Each of the filter blocks 110 a-d includes a plurality of cross-talkcancellation filters which may be implemented as least-squares (LS)filters. The filter transfer functions for the cross-talk cancellationfilters are determined according to:G=H ⁻¹ ×T

where,

G is a matrix representing the filter transfer functions that we aresolving for;

H is a matrix representing the measured acoustic transfer functions, andH⁻¹ is the pseudo inversion of that matrix; and

T is matrix representing the target that we are trying to achieve. Thesetargets are set to define the desired cross-talk cancellation at thevehicle occupant's heads.

For the example illustrated in FIG. 1, the filter transfer functions forthe cross-talk cancellation filters in the first and second filterblocks 110 a, 110 b are solved for together since those two filterblocks work together to provide cross-talk cancellers at front seatingpositions and to cancel audio from the front zone at the rear seatingpositions. Similarly, the filter transfer functions for the cross-talkcancellation filters in the third and fourth filter blocks 110 c, 110 dare solved for together since those two filter blocks work together toprovide cross-talk cancellers in the rear seating positions and tocancel audio from the rear zone at the front seating positions. Thecross-talk cancellation filter blocks 110 a-d provide respectivefiltered audio signals 130, 132, 134, 136 to corresponding sets of theheadrest mounted speakers which transduce the filtered audio signals130, 132, 134, 136.

The system 100 includes a pair of front headrests 140, 142 and a pair ofrear headrests 144, 146. Each of the front headrests is provided withfour electro-acoustic transducers including two forward firingelectro-acoustic transducers 148 a, 148 b, 150 a, 150 b and two rearfiring electro-acoustic transducers 152 a, 152 b, 154 a, 154 b. Theforward firing speakers 148 a, 148 b, 150 a, 150 b of the frontheadrests 140, 142 provide audible audio content for the occupants inthe first, front zone 101 a (i.e., the two front seats), while alsoassisting in enabling inter-aural cross-talk cancellation in each of thetwo front seats, and inter-seat cross-talk cancellation between the twofront seats. The rear firing speakers 152 a, 152 b, 154 a, 154 b of thefront headrests 144, 146 assist in enabling inter zone cross-talkcancellation between the front and rear zones 101 a, 101 b.

Each of the rear headrests 144, 146 include two forward firing speakers156 a, 156 b, 158 a, 158 b. The forward firing speakers 156 a, 156 b,158 a, 158 b of the rear headrests 144, 146 provide audible audiocontent for the occupants in the second, rear zone 101 b (i.e., the tworear seats), while also assisting in enabling inter-aural cross-talkcancellation in each of the two rear seats, and inter-seat cross-talkcancellation between the two rear seats.

Referring to FIG. 2A, the first cross-talk cancellation filter block 110a includes a plurality of cross-talk cancellation filters (eight shown).The first amplitude adjusted audio signal 126, shown as a stereo audiosignal consisting of left and right audio channels 126 a, 126 b, ispassed through the first cross-talk cancellation filter block 110 a toproduce first filtered audio signals 130 a-d (collectively referenced as130), one for each of the forward firing speakers 148 a, 148 b, 150 a,150 b in the front headrests 140, 142. These filtered audio signals 130determine the net acoustic energy associated with each acoustic channelin the first audio signal 118 that is provided to the occupants D0, D1in the front seats.

A left channel filter 200 _(L1) associated with a forward firing leftspeaker 148 a of the driver's headrest 140 modifies the left channelinput signal 126 a taking into account the acoustic transfer functionsfrom each of the other front headrest mounted speakers 148 b, 150 a, 150b, 152 a, 152 b, 154 a, 154 b to an expected position of the driver'sleft ear to produce a first output signal component that is configuredto reproduce the left channel acoustic content of the first audio signalat the driver's left ear.

A right channel filter 200 _(R1) associated with the forward firing leftspeaker 148 a of the driver's headrest 140 modifies the right channelinput 126 b of the first amplitude adjusted audio signal 126 taking intoaccount the transfer functions from each of the other front headrestmounted speakers 148 b, 150 a, 150 b, 152 a, 152 b, 154 a, 154 b to theexpected position of the driver's left ear to produce a second outputsignal component that is configured to cancel the right channel acousticcontent of the first audio signal 118 that is leaked to the driver'sleft ear from the other speakers 148 b, 150 a, 150 b, 152 a, 152 b, 154a, 154 b in the front headrests 140, 142.

The first and second output signal components are combined to produce afiltered audio signal 130 a which is provided to the forward firing leftspeaker 148 a in the driver's headrest 140. The remaining cross-talkcancellation filters of the first cross-talk cancellation filter block110 a and the associated speakers 148 b, 150 a, 150 b operate similarlyso that the front seat occupants D0, D1 hear only left audio content ofthe first audio signal 118 at their respective left ears and hear onlyright audio content of the first audio signal 118 at their respectiveright ears.

Filters 200 _(L2) and 200 _(R2) provide a filtered audio signal 130 b tothe forward firing right speaker 148 b in the driver's headrest 140,which is transduced to reproduce the right channel acoustic content ofthe first audio signal 118 at the driver's right ear, while cancellingleft channel content of the first audio signal 118 leaked by the otherfront headrest mounted speakers 148 a, 150 a, 150 b, 152 a, 152 b, 154a, 154 b at the driver's right ear.

Filters 200 _(L3) and 200 _(R3) provide a filtered audio signal 130 c tothe forward firing left speaker 150 a in the front passenger's headrest142, which is transduced to reproduce the left channel acoustic contentof the first audio signal 118 at the passenger's left ear, whilecancelling right channel content of the first audio signal 118 leaked bythe other front headrest mounted speakers 148 a, 148 b, 150 b, 152 a,152 b, 154 a, 154 b at the passenger's left ear.

Filters 200 _(L4) and 200 _(R4) provide a filtered audio signal 130 d tothe forward firing right speaker 150 b in the front passenger's headrest142, which is transduced to reproduce the right channel acoustic contentof the first audio signal 118 at the passenger's right ear, whilecancelling left channel content of the first audio signal 118 leaked bythe other front headrest mounted speakers 148 a, 148 b, 150 a, 152 a,152 b, 154 a, 154 b at the passenger's right ear.

Referring to FIG. 2B, the second cross-talk cancellation filter block110 b includes a plurality of cross-talk cancellation filters (eightshown). The first amplitude adjusted audio signal 126, shown again as astereo audio signal consisting of left and right audio channels 126 a,126 b, is passed through the second cross-talk cancellation filter block110 b to produce second filtered audio signals 132 a-d (collectivelyreferenced as 132), one for each of the rear firing speakers 152 a, 152b, 154 a, 154 b in the front headrests 140, 142. These filtered audiosignals 132 determine the net acoustic energy associated with eachacoustic channel in the first audio signal 118 that is provided to theoccupants D2, D3 in the rear seats.

A left channel filter 202 _(L1) associated with a rear firing leftspeaker 152 a of the driver's headrest 140 modifies the left channelinput signal 126 a taking into account the acoustic transfer functionsfrom each of the other front headrest mounted speakers 148 a, 148 b, 150a, 150 b, 152 b, 154 a, 154 b to an expected position of the rear leftpassenger's left ear to produce a first output signal component that isconfigured to cancel the left channel acoustic content of the firstaudio signal 118 that is leaked to the rear left passenger's left earfrom the other front headrest mounted speakers 148 a, 148 b, 150 a, 150b, 152 b, 154 a, 154 b.

A right channel filter 202R₁ associated with the rear firing leftspeaker 152 a of the driver's headrest modifies the right channel inputfrom first amplitude adjusted audio signal 126 b taking into account theacoustic transfer functions from each of the other front headrestmounted speakers 148 a, 148 b, 150 a, 150 b, 152 b, 154 a, 154 b to theexpected position of the rear left passenger's left ear to produce asecond output signal component that is configured to cancel the rightchannel acoustic content of the first audio signal 118 that is leaked tothe rear left passenger's left ear from the other front headrest mountedspeakers 148 a, 148 b, 150 a, 150 b, 152 b, 154 a, 154 b.

The first and second output signal components are combined to produce afiltered audio signal 132 a which is provided to the rear firing leftspeaker 152 a in the driver's headrest 140. The remaining cross-talkcancellation filters of the second cross-talk cancellation filter block110 b and the associated speakers 152 b, 154 a, 154 b operate similarlyso that audio content from the first audio signal 118 is cancelled atthe seating positions in the rear listening zone 101 b (FIG. 1).

Filters 202 _(L2) and 202 _(R2) provide a filtered audio signal 132 b tothe rear firing right speaker 152 b in the driver's headrest 140, whichis transduced to cancel audio content of the first audio signal 118leaked by the other front headrest mounted speakers 148 a, 148 b, 150 a,150 b, 152 a, 154 a, 154 b at the rear left passenger's right ear.

Filters 202 _(L3) and 202 _(R3) provide a filtered audio signal 132 c tothe rear firing left speaker 154 a in the front passenger's headrest142, which is transduced to cancel audio content of the first audiosignal 118 leaked by the other front headrest mounted speakers 148 a,148 b, 150 a, 150 b, 152 a, 152 b, 154 b at the rear right passenger'sleft ear.

Filters 202 _(L4) and 202 _(R4) provide a filtered audio signal 132 d tothe rear firing right speaker 154 b in the front passenger's headrest142, which is transduced to cancel audio content of the first audiosignal 118 leaked by the other front headrest mounted speakers 148 a,148 b, 150 a, 150 b, 152 a, 152 b, 154 a at the rear right passenger'sright ear.

Referring to FIG. 2C, the third cross-talk cancellation filter block 110c includes a plurality of cross-talk cancellation filters (eight shown).The second amplitude adjusted audio signal 128, shown as a stereo audiosignal consisting of left and right audio channels 128 a, 128 b, ispassed through the third cross-talk cancellation filter block 110 c toproduce third filtered audio signals 134 a-d (collectively referenced as134), one for each of the forward firing speakers 148 a, 148 b, 150 a,150 b in the front headrests 140, 142. These filtered audio signals 134determine the net acoustic energy associated with each acoustic channelin the second audio signal 120 that is provided to the occupants in thefront seats.

A left channel filter 204 _(L1) associated with a forward firing leftspeaker 148 a of the driver's headrest 140 modifies the left channelinput signal 128 a taking into account the acoustic transfer functionsfrom each of the rear headrest mounted speakers 156 a, 156 b, 158 a, 158b (FIG. 1) and from each of the other forward firing front headrestmounted speakers 148 b, 150 a, 150 b to an expected position of thedriver's left ear to produce a first output signal component that isconfigured to cancel the left channel acoustic content of the secondaudio signal 120 that is leaked to the driver's left ear from the rearheadrest mounted speakers 156 a, 156 b, 158 a, 158 b and from the otherforward firing front headrest mounted speakers 148 b, 150 a, 150 b.

A right channel filter 204 _(R1) associated with the forward firing leftspeaker 148 a of the driver's headrest 140 modifies the right channelinput 128 b from the second amplitude adjusted audio signal 128 takinginto account the acoustic transfer functions from each of the rearheadrest mounted speakers 156 a, 156 b, 158 a, 158 b and from each ofthe other forward firing front headrest mounted speakers 148 b, 150 a,150 b to the expected position of the driver's left ear to produce asecond output signal component that is configured to cancel the rightchannel acoustic content of the second audio signal 120 that is leakedto the driver's left ear from the rear headrest mount speakers 156 a,156 b, 158 a, 158 b and from the other forward firing front headrestmounted speakers 148 b, 150 a, 150 b.

The first and second output signal components are combined to produce afiltered audio signal 134 a which is provided to the front firing leftspeaker 148 a in the driver's headrest 140. The remaining cross-talkcancellation filters of the third cross-talk cancellation filter block110 c and the associated speakers 148 b, 150 a, 150 b operate similarlyso that audio content from the second audio signal 120 is cancelled atthe seating positions in the front listening zone 101 a (FIG. 1).

Filters 204 _(L2) and 204 _(R2) provide a filtered audio signal 134 b tothe front firing right speaker 148 b in the driver's headrest 140, whichis transduced to cancel audio content of the second audio signal 120leaked by the other front headrest mounted speakers 148 a, 150 a, 150 band the rear headrest mounted speakers 156 a, 156 b, 158 a, 158 b at thedriver's right ear.

Filters 204 _(L3) and 204 _(R3) provide a filtered audio signal 134 c tothe front firing left speaker 150 a in the front passenger's headrest142, which is transduced to cancel audio content of the second audiosignal 120 leaked by the other front headrest mounted speakers 148 a,148 b, 150 b and the rear headrest mounted speakers 156 a, 156 b, 158 a,158 b at the front passenger's left ear.

Filters 204 _(L4) and 204 _(R4) provide a filtered audio signal 134 d tothe front firing right speaker 150 b in the front passenger's headrest142, which is transduced to cancel audio content of the second audiosignal 120 leaked by the other front headrest mounted speakers 148 a,148 b, 150 a and the rear headrest mounted speakers 156 a, 156 b, 158 a,158 b at the front passenger's right ear.

Referring to FIG. 2D, the fourth cross-talk cancellation filter 110 dblock includes a plurality of cross-talk cancellation filters (eightshown). The second amplitude adjusted audio signal 128, shown again as astereo audio signal consisting of left and right audio channels 128 a,128 b, is passed through the fourth cross-talk cancellation filter block110 d to produce fourth filtered audio signals 136 a-d (collectivelyreferenced as 136), one for each of the speakers 156 a, 156 b, 158 a,158 b in the rear headrests 144, 146. These filtered audio signals 136determine the net acoustic energy associated with each acoustic channelin the second audio signal 120 that is provided to the occupants in therear seats.

A left channel filter 206L₁ associated with a left speaker 156 a of therear left passenger's headrest 144 modifies the left channel inputsignal 128 a taking into account the acoustic transfer functions fromeach of the other rear headrest mounted speakers 156 b, 158 a, 158 b andthe forward firing speakers 148 a, 148 b, 150 a, 150 b (FIG. 1) of thefront headrests 140, 142 (FIG. 1) to an expected position of the rearleft passenger's left ear to produce a first output signal componentthat is configured to reproduce the left channel acoustic content of thesecond audio signal 120 at the rear left passenger's left ear.

A right channel filter 206R₁ associated with the left speaker 156 a ofthe rear left passenger's headrest 144 modifies the right channel input128 b from second amplitude adjusted audio signal 128 taking intoaccount the acoustic transfer functions from each of the other rearheadrest mounted speakers 156 b, 158 a, 158 b and the forward firingspeakers 148 a, 148 b, 150 a, 150 b of the front headrests 140, 142 tothe expected position of the rear left passenger's left ear to produce asecond output signal component that is configured to cancel the rightchannel acoustic content of the second audio signal 120 that is leakedto the rear left passenger's left ear from the other speakers in therear headrests 156 b, 158 a, 158 b and from the forward firing speakers148 a, 148 b, 150 a, 150 b mounted in the front headrests 140, 142.

The first and second output signal components are combined to produce afiltered audio signal 136 a which is provided to the left speaker 156 ain the rear left passenger's headrest 144. The remaining cross-talkcancellation filters of the fourth cross-talk cancellation filter block110 d and the associated speakers 156 b, 158 a, 158 b operate similarlyso that the rear seat occupants hear only left audio content of thesecond audio signal 120 at their respective left ears and hear onlyright audio content of the second audio signal 120 at their respectiveright ears.

Filters 206 _(L2) and 206 _(R2) provide a filtered audio signal 136 b tothe right speaker 156 b in the rear left passenger's headrest 144, whichis transduced to reproduce the right channel acoustic content of thesecond audio signal 120 at the rear left passenger's right ear, whilecancelling left channel content of the second audio signal 120 leaked bythe forward firing front headrest mounted speakers 148 a, 148 b, 150 a,150 b and the other rear headrest mounted speakers 156 b, 158 a, 158 bat the rear left passenger's right ear.

Filters 206 _(L3) and 206 _(R3) provide a filtered audio signal 136 c tothe left speaker 158 a in the rear right passenger's headrest 146, whichis transduced to reproduce the left channel acoustic content of thesecond audio signal 120 at the rear right passenger's left ear, whilecancelling right channel content of the second audio signal 120 leakedby the forward firing front headrest mounted speakers 148 a, 148 b, 150a, 150 b and the other rear headrest mounted speakers 156 a, 156 b, 158b at the rear right passenger's left ear.

Filters 206 _(L4) and 206 _(R4) provide a filtered audio signal 136 d tothe forward firing right speaker 158 b in the rear right passenger'sheadrest 146, which is transduced to reproduce the right channelacoustic content of the second audio signal 120 at the rear rightpassenger's right ear, while cancelling left channel content of thesecond audio signal 120 leaked by the forward firing front headrestmounted speakers 148 a, 148 b, 150 a, 150 b and the other rear headrestmounted speakers 156 a, 156 b, 158 a at the rear right passenger's rightear.

The above described audio system can allow rear vehicle occupants (a/k/arear passengers), i.e., occupants in the rear seats, to listen todifferent audio content than the occupants in the front seats. Thesystem can also allow both sets of occupants (i.e., front and back) tolisten to the same audio content at contrasting volumes level. Forexample, passengers in the back seats may wish to listen to the sameaudio content as the occupants in the front seat, but at a low volumelevel.

When the volume difference between zones becomes large (>˜6 dB), theremay be some spectral coloring in the attenuated zone (i.e., the lowervolume zone) because of the relatively poorer isolation at higherfrequencies. This may be particularly noticeable when the same audiocontented is presented in both listening zones. In some cases, toinhibit such spectral coloring, lower frequencies may be attenuated lessthan higher frequencies in the attenuated zone, which can help toflatten the acoustic energy in the attenuated zone (i.e., to maintain asubstantially balanced spectrum) to provide a user experience that feelsmore like regular volume control.

FIG. 3 illustrates an implementation of the audio system which providesvolume-based equalization to account for band-dependent isolation. Thesystem 300 of FIG. 3 includes volume-based equalization circuitry 302 a,302 b for the front and rear zones, respectively. The volume-basedequalization circuitry 302 a, 302 b receives a control signal from thecontrol circuitry 114.

In response to volume control information received from a user throughmanual input, the control circuitry 114 sends a volume control signal304 to the front and rear volume-based equalization circuitry 302 a, 302b, respectively. The volume control signal 304 includes an indication ofthe respective volume settings for both the front zone 101 a and therear zone 101 b. The volume-based equalization circuitry 302 a, 302 bcan then use this information to dynamically adjust equalization toinhibit spectral coloring when the relative volumes of the zones exceedsa predetermined threshold.

Referring to FIG. 4, the volume-based equalization circuitry 302 a, 302b (collectively referenced as “302”) includes a low pass filter 306,which filters high frequency content out of the audio signal 118, 120and passes a low frequency signal 308 including the low frequencycontent to a low frequency dynamic gain 310. The volume-basedequalization circuitry 302 also include a high pass filter 312 whichfilters low frequency content out of the audio signal 118, 120 andpasses a high frequency signal 314 including the high frequency contentto a high frequency dynamic gain 316.

Gain determination logic 318 receives the volume control signal 304 fromthe control circuitry 114 (FIG. 3) and may adjust the low and/or highfrequency dynamic gains 310, 316 accordingly. In particular, the gaindetermination logic 318 determines the difference in the volume settingsbetween the associated zone (e.g., rear zone 101 b) and the other zone(e.g., front zone 101 a) and may adjust the respective gains/attenuationapplied by the low and high frequency dynamic gains 310, 316 based onthe determined difference. In particular, the gain determination logic318 may adjust the low frequency dynamic gain 310 so as to limit theattenuation of the low frequencies when the determined volumedifferential exceeds a first predetermined level (e.g., 6 dB).

In some cases, the attenuation of the lower frequencies may be limitedto that which is attainable for the higher frequencies. For example, ifthe determined volume differential is −20 dB, but the system is onlyexpected to provide up to −15 dB attenuation at the higher frequenciesdue to the leakage of such frequencies from the other zone, then theattenuation of the lower frequencies may be limited to −15 dB.

FIG. 5 illustrates exemplary attenuation curves that may be employed bythe gain determination logic in setting the gain. At −5 dB differentialbetween front and rear zone volume settings the attenuation curve 502 issubstantially flat across all frequencies because good cancellation isgenerally expected in that range. Curve 504 represents a −10 dBdifferential where the attenuation of the lower frequencies is slightlylimited to account for the limited ability of the system to attenuate atthe higher frequencies. Curve 506 shows a more pronounced limiting ofthe attenuation of the lower frequencies at a −20 dB volume settingdifferential.

In some cases, the attenuation limiting may be turned off when thevolume setting differential exceeds a second predetermined value underthe assumption that the user is more interested in maximum attenuationthan maintaining a balanced spectrum.

Since the attenuation limiting is most beneficial when the two zones arereproducing the same audio content, the system may restrict applicationof the attenuation limiting to situations in which both the zones areproviding the same audio content.

The acoustic transfer functions within the vehicle cabin 103 (FIGS. 1 &3) can change with vehicle occupancy. In some cases, the system may beconfigured to use input indicative of vehicle occupancy to dynamicallyadjust the coefficients of the filters thereby to take into account thechange in the acoustic transfer functions. For example, FIG. 6illustrates an implementation of a vehicle audio system 600 whichincludes a filter coefficient determination module 602 which utilizesinput 603 indicative of the vehicle occupancy to set the coefficients ofthe filters. These coefficients may be predetermined based on transferfunction measurements taken with varying occupancy configurations. Thecoefficients for the different occupancy configurations may be stored ina look-up table accessible to the coefficient determination module 602.

The input 603 indicative of vehicle occupancy may be obtained fromsensors in the individual seats which detect the presence of apassenger. In some cases, this information may be obtained from thevehicle controller area network (CAN) bus. The sensor input may beobtained via the control circuitry 114 and relayed to the filtercoefficient determination module 602, as illustrated in FIG. 6.

While various concepts have been described above with particularreference to a two zone configuration with the zones arranged one infront of the other, the concepts also be applicable to otherconfigurations. For example, FIG. 7 illustrates an implementation of avehicle audio system 700 in which separate listening zones (i.e., leftand right listening zones 701 a, 701 b) are provided for twoside-by-side seating locations within a vehicle cabin 703.

The system includes one or more audio sources 702 which are coupled toaudio signal processing circuitry 704. The audio signal processingcircuitry 704 is coupled to left and right volume adjustment circuitry706 a, 706 b, respectively. The left and right volume adjustmentcircuitry 706 a, 706 b is coupled to headrest mounted speakers viacross-talk cancellation filter blocks 710 a, 710 b.

In response to control information received from a user through manualinput, control circuitry 714 sends a signal 716 to the audio signalprocessing circuitry 704 selecting a given audio source 702 for the leftand right listening zones 701 a, 701 b. That is, the signal identifieswhich audio source 702 is selected for each of the listening zones 701a, 701 b. Each listening zone 701 a, 701 b can select a different audiosource, or a common audio source may be selected for both of the left(driver) and right (passenger) listening zones 701 a, 701 b.

The audio signal processing circuitry 704 delivers a first audio signal718 representing audio content for the left zone 701 a to left volumeadjustment circuitry 706 a, and delivers a second audio signal 720representing audio content for the right zone 701 b to right volumeadjustment circuitry 706 b.

In response to volume control information received from a user throughmanual input, the control circuit sends first and second volume controlsignals 722, 724 to the left and right volume adjustment circuitry 706a, 706 b, respectively. The left and right volume adjustment circuitry706 a, 706 b adjust the respective amplitudes of the audio signals 718,720 in response to the volume control signals 722, 724 and provide firstand second amplitude adjusted audio signals 726, 728 to the cross-talkcancellation filter blocks 710 a, 710 b, respectively. In that regard,the left volume adjustment circuitry 706 a controls volume of audiocontent presented in the left listening zone 701 a, and the right volumeadjustment circuitry 706 b operates to control the volume of audiocontent presented in the right listening zone 701 b. Consequently, evenwhen the same audio content is selected for presentation in both zones,the volume level may still differ between the zones.

In the illustrated example, the left volume adjustment circuitry 706 aprovides a first amplitude adjusted audio signal 726 to the firstcross-talk cancellation filter block 710 a, and the right volumeadjustment circuitry 706 b provides a second amplitude adjusted audiosignal 728 to the second cross-talk cancellation filter block 710 b.

Referring to FIG. 8A, the first cross-talk cancellation filter block 710a includes a plurality of cross-talk cancellation filters (eight shown).The first amplitude adjusted audio signal 726, shown as a stereo audiosignal consisting of left and right audio channels 726 a, 726 b, ispassed through the first cross-talk cancellation filter block 710 a toproduce first filtered audio signals 730 a, 730 b, 732 a, 732 b, one foreach of the headrest mounted speakers. These filtered audio signalsdetermine the net acoustic energy associated with each acoustic channelin the first audio signal 718 that is provided to the occupants D0, D1in the seats.

A left channel filter 800 _(L1) associated with a left speaker 748 a ofthe driver's headrest 740 modifies the left channel input signal 726 ataking into account the acoustic transfer functions from each of theother headrest mounted speakers 748 b, 750 a, 750 b to an expectedposition of the driver's left ear to produce a first output signalcomponent that is configured to reproduce the left channel acousticcontent of the first audio signal 718 at the driver's left ear.

A right channel filter 800 _(R1) associated with the left speaker 748 aof the driver's headrest 740 modifies the right channel input 726 b fromthe first amplitude adjusted audio signal 726 taking into account theacoustic transfer functions from each of the other headrest mountedspeakers 748 b, 750 a, 750 b to the expected position of the driver'sleft ear to produce a second output signal component that is configuredto cancel the right channel acoustic content of the first audio signal718 that is leaked to the driver's left ear from the other speakers 748b, 750 a, 750 b in the headrests 740, 742.

The first and second output signal components are combined to produce afiltered audio signal 730 a which is provided to the left speaker 748 ain the driver's headrest 740. The remaining cross-talk cancellationfilters of the first cross-talk cancellation filter block 710 a and theassociated speakers 748 b, 750 a, 750 b operate similarly so that theoccupant in the driver's seat hears only left audio content of the firstaudio signal 718 at his/her left ear and hears only right audio contentof the first audio signal 718 at his/her right ear.

Filters 800 _(L2) and 800 _(R2) provide a filtered audio signal 730 b tothe right speaker 748 b in the driver's headrest 740, which istransduced to reproduce the right channel acoustic content of the firstaudio signal 718 at the driver's right ear, while cancelling leftchannel content of the first audio signal 718 leaked by the otherheadrest mounted speakers 748 a, 750 a, 750 b at the driver's right ear.

Filters 800 _(L3) and 800 _(R3) provide a filtered audio signal 732 a tothe left speaker 750 a in the passenger's headrest 742, which istransduced to cancel left and right channel content of the first audiosignal 718 leaked by the other headrest mounted speakers 748 a, 748 b,750 b at the passenger's left ear.

Filters 800 _(L4) and 800 _(R4) provide a filtered audio signal 732 b tothe right speaker 750 b in the passenger's headrest 742, which istransduced to cancel the left and right channel content of the firstaudio signal 718 leaked by the other headrest mounted speakers 748 a,748 b, 750 a at the passenger's right ear.

Referring to FIG. 8B, the second cross-talk cancellation filter block710 b includes a plurality of cross-talk cancellation filters (eightshown). The second amplitude adjusted audio signal 728, shown as astereo audio signal consisting of left and right audio channels 728 a,728 b, is passed through the second cross-talk cancellation filter block710 b to produce second filtered audio signals 734 a, 734 b, 736 a, 736b, one for each of the headrest mounted speakers 748 a, 748 b, 750 a,750 b. These filtered audio signals 734 a, 734 b, 736 a, 736 b determinethe net acoustic energy associated with each acoustic channel in thesecond audio signal 720 that is provided to the seat occupants D0, D1.

A left channel filter 802 _(L1) associated with the left speaker 748 aof the driver's headrest 740 modifies the left channel input signal 728a taking into account the acoustic transfer functions from each of theother headrest mounted speakers 748 b, 750 a, 750 b to an expectedposition of the driver's left ear to produce a first output signalcomponent that is configured to cancel the left channel acoustic contentof the second audio signal 720 that is leaked to the driver's left earfrom the other headrest mounted speakers 748 b, 750 a, 750 b.

A right channel filter 802 _(R1) associated with the driver's headrest740 modifies the right channel input signal 728 b from the secondamplitude adjusted audio signal 728 taking into account the acoustictransfer functions from each of the other front headrest mountedspeakers 748 b, 750 a, 750 b to the expected position of the rear leftpassenger's left ear to produce a second output signal component that isconfigured to cancel the right channel acoustic content of the secondaudio signal 720 that is leaked to the driver's left ear from the otherfront headrest mounted speakers 748 b, 750 a, 7540 b. The first andsecond output signal components are combined to produce a filtered audiosignal 734 a, which is provided to the left speaker 748 a in thedriver's headrest 740.

Filters 802 _(L2) and 802 _(R2) provide a filtered audio signal 734 b tothe right speaker 748 b in the driver's headrest 740, which istransduced to cancel audio content of the second audio signal 720 leakedby the other headrest mounted speakers 748 a, 750 a, 750 b at thedriver's right ear.

Filters 802 _(L3) and 803 _(R3) provide a filtered audio signal 736 a tothe left speaker 750 a in the passenger's headrest 742, which istransduced to reproduce the left channel acoustic content of the secondaudio signal 720 at the passenger's left ear, while cancelling rightchannel content of the second audio signal 720 leaked by the otherheadrest mounted speakers 748 a, 748 b, 750 b at the passenger's leftear.

Filters 802 _(L4) and 802 _(R4) provide a filtered audio signal 736 b tothe right speaker 750 b in the passenger's headrest 742, which istransduced to reproduce the right channel acoustic content of the secondaudio signal 720 at the passenger's right ear, while cancelling leftchannel content of the second audio signal 720 leaked by the otherheadrest mounted speakers 748 a, 748 b, 750 a at the passenger's rightear.

FIG. 9 illustrates an implementation of a vehicle audio system 900 withside-by-side listening zones 701 a, 701 b which includes volume-basedequalization circuitry 902 a, 902 b for limiting attenuation of lowfrequencies when the volume differential in the two listening zones 701a, 701 b exceeds a first predetermined value. The volume-basedequalization circuitry may operate in a similar manner to that discussedabove with reference to FIGS. 3 & 4.

FIG. 9 also shows an optional filter coefficient determination module902 which may utilize input 903 indicative of the vehicle occupancy toset the coefficients of the filters. These coefficients may bepredetermined based on acoustic transfer function measurements takenwith varying occupancy configurations. The coefficients for thedifferent occupancy configurations may be stored in a look-up tableaccessible to the coefficient determination module 902.

Although a few implementations have been described in detail above,other modifications are possible. While a particular example ofvolume-based equalization circuitry was described above with respect toFIG. 4, FIG. 10 illustrates another implementation that uses one pathwith the low pass filter (306), but instead of a high pass filter on theother path, another type of filter 1000 may be provided. In some cases,the other filter 1000 may be an all pass filter. For example, the lowpass filter 306, which has non-zero phase, may cancel at certainfrequencies with the second gain path 1002, which in this example is abroadband gain path. Adding an all pass filter to the second gain path1002 can help to match the phase of the low pass filter 306 to helpavoid the unintended cancellation. Alternatively or additionally, otherfilters types may be applied along the second gain path 1002, or alongadditional gain paths (not shown), so that the entire frequency spectrumis adjustable.

Alternatively or additionally, in some cases, just a straight gain (nofilter) is provided along one of the signal paths. Such animplementation may also be designed to provide the results shown in FIG.5.

Furthermore, although the foregoing description focused on theapplication of attenuation limiting to low frequencies because those arethe most obvious spectral flaw when one zone is attenuated, in somecases, this mechanism may alternatively or additionally be utilized tosmooth out mid- and/or high-frequencies.

In addition, as an alternative or in addition to limiting attenuation ofa frequency portion within the attenuated zone, the amplification offrequencies in one or more frequency portions (e.g., mid- and/orhigh-frequency portions) of the audio signal 118, 120 (FIG. 3) may belimited in the amplified zone, thereby to limit bleed/leakage of thosefrequencies into the attenuated zone. For example, referring back toFIG. 4, the gain determination logic 318 may adjust the high frequencydynamic gain 316 so as to limit the amplification of frequencies in ahigh frequency portion of the audio signal 118, 120, when the determinedvolume differential exceeds a first predetermined level (e.g., 6 dB).

Other near-field speaker configurations are possible. For example, thespeakers may also be mounted on visors or in a vehicle headlinerproximal to seating locations.

A number of implementations have been described. Nevertheless, it willbe understood that additional modifications may be made withoutdeparting from the scope of the inventive concepts described herein,and, accordingly, other implementations are within the scope of thefollowing claims.

What is claimed is:
 1. An audio system comprising: a plurality ofnear-field speakers arranged in a listening area; a plurality ofcross-talk cancellation filters coupled to the near-field speakers, thenear-field speakers and the cross-talk cancellation filters beingarranged to provide first and second listening zones in the listeningarea in which audio from the first listening zone is cancelled in thesecond listening zone and vice versa; at least one audio source forproviding identical audio content to both the first listening zone andthe second listening zone; first volume-based equalization circuitryconfigured to receive a first audio signal representing audio contentfor the first listening zone from the at least one audio source, and tocontrol a volume adjustment applied to the first audio signal thereby tocontrol a volume of audio in the first listening zone based on a firstvolume setting; and second volume-based equalization circuitryconfigured to receive a second audio signal representing audio contentfor the second listening zone, and to control a volume adjustmentapplied to the second audio signal thereby to control a volume of audioin the second listening zone based on a second volume setting, whereinthe first volume-based equalization circuitry is configured to determinewhether a volume setting differential between the first volume settingfor the first listening zone and the second volume setting for thesecond listening zone exceeds a predetermined threshold, and if so,limit attenuation of a first frequency portion of the first audio signalto a corresponding amount of attenuation of a second frequency portionof the first audio signal, thereby to achieve a desired volume of audioin the first listening zone based on the first volume setting whilemaintaining a substantially balanced spectrum in the first listeningzone, wherein the amount of attenuation of the second frequency portionof the first audio signal depends on an amount of leakage of a secondfrequency portion of the second audio signal from the second listeningzone into the first listening zone.
 2. The audio system of claim 1,wherein the second volume-based equalization circuitry is configured tolimit amplification of the second frequency portion of the second audiosignal when the volume setting differential exceeds the predeterminedvalue.
 3. The audio system of claim 1, wherein the first frequencyportion of the first audio signal is a low frequency portion and thesecond frequency portions of both the first audio signal and the secondaudio signal is a high frequency portion.
 4. The audio system of claim1, further comprising control circuitry configured to provide the firstand second volume-based equalization circuitry with a volume controlsignal, wherein the first volume-based equalization circuitry isconfigured to determine the volume setting differential based oninformation provided in the volume control signal.
 5. The audio systemof claim 1, wherein the first volume-based equalization circuitry onlylimits the attenuation of the first frequency portion of the first audiosignal when the volume setting differential exceeds the predeterminedthreshold and the first and second audio signals represent the sameaudio content.
 6. The audio system of claim 1, wherein the firstfrequency portion of the first audio signal is a low frequency portionand the second frequency portion of the second audio signal is a highfrequency portion, and wherein the amount of leakage of the secondfrequency portion of the second audio signal from the second listeningzone into the first listening zone depends on an amount of cancelationthat is attainable for the high frequency portion of the second audiosignal in the first listeing zone.